Subject: [PATCH] 9p: fix crash when transaction killed

From: Schspa Shi <[email-masked]>
Date: Wed, 30 Nov 2022 00:22:51 +0800

The transport layer of fs does not fully support the cancel request.
When the request is in the REQ_STATUS_SENT state, p9_fd_cancelled
will forcibly delete the request, and at this time p9_[read/write]_work
may continue to use the request. Therefore, it causes UAF .

There is the logs from syzbot.

Corrupted memory at 0xffff88807eade00b [ 0xff 0x07 0x00 0x00 0x00 0x00
0x00 0x00 . . . . . . . . ] (in kfence-#110):
 p9_fcall_fini net/9p/client.c:248 [inline]
 p9_req_put net/9p/client.c:396 [inline]
 p9_req_put+0x208/0x250 net/9p/client.c:390
 p9_client_walk+0x247/0x540 net/9p/client.c:1165
 clone_fid fs/9p/fid.h:21 [inline]
 v9fs_fid_xattr_set+0xe4/0x2b0 fs/9p/xattr.c:118
 v9fs_xattr_set fs/9p/xattr.c:100 [inline]
 v9fs_xattr_handler_set+0x6f/0x120 fs/9p/xattr.c:159
 __vfs_setxattr+0x119/0x180 fs/xattr.c:182
 __vfs_setxattr_noperm+0x129/0x5f0 fs/xattr.c:216
 __vfs_setxattr_locked+0x1d3/0x260 fs/xattr.c:277
 vfs_setxattr+0x143/0x340 fs/xattr.c:309
 setxattr+0x146/0x160 fs/xattr.c:617
 path_setxattr+0x197/0x1c0 fs/xattr.c:636
 __do_sys_setxattr fs/xattr.c:652 [inline]
 __se_sys_setxattr fs/xattr.c:648 [inline]
 __ia32_sys_setxattr+0xc0/0x160 fs/xattr.c:648
 do_syscall_32_irqs_on arch/x86/entry/common.c:112 [inline]
 __do_fast_syscall_32+0x65/0xf0 arch/x86/entry/common.c:178
 do_fast_syscall_32+0x33/0x70 arch/x86/entry/common.c:203
 entry_SYSENTER_compat_after_hwframe+0x70/0x82

Below is a similar scenario, the scenario in the syzbot log looks more
complicated than this one, but the root cause seems to be the same.

     T21124               p9_write_work        p9 read_work
======================== first trans =================================
p9_client_walk
  p9_client_rpc
    p9_client_prepare_req
    /* req->refcount == 2 */
    c->trans_mod->request(c, req);
      p9_fd_request
        req move to unsent_req_list
                            req->status = REQ_STATUS_SENT;
                            req move to req_list
                            << send to server >>
    wait_event_killable
    << get kill signal >>
    if (c->trans_mod->cancel(c, req))
       p9_client_flush(c, req);
         /* send flush request */
         req = p9_client_rpc(c, P9_TFLUSH, "w", oldtag);
		 if (c->trans_mod->cancelled)
            c->trans_mod->cancelled(c, oldreq);
              /* old req was deleted from req_list */
              /* req->refcount == 1 */
  p9_req_put
    /* req->refcount == 0 */
    << preempted >>
                                       << get response, UAF here >>
                                       m->rreq = p9_tag_lookup(m->client, m->rc.tag);
                                         /* req->refcount == 1 */
                                       << do response >>
                                       p9_client_cb(m->client, m->rreq, REQ_STATUS_RCVD);
                                         /* req->refcount == 0 */
                                         p9_fcall_fini
                                         /* request have been freed */
    p9_fcall_fini
     /* double free */
                                       p9_req_put(m->client, m->rreq);
                                         /* req->refcount == 1 */

To fix it, we can wait the request with status REQ_STATUS_SENT returned.

Reported-by: [email-masked]

Signed-off-by: Schspa Shi <[email-masked]>
---
 net/9p/client.c   |  2 +-
 net/9p/trans_fd.c | 12 ++++++++++++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/net/9p/client.c b/net/9p/client.c
index aaa37b07e30a..963cf91aa0d5 100644
--- a/net/9p/client.c
+++ b/net/9p/client.c
@@ -440,7 +440,7 @@ void p9_client_cb(struct p9_client *c, struct p9_req_t *req, int status)
 	smp_wmb();
 	req->status = status;
 
-	wake_up(&req->wq);
+	wake_up_all(&req->wq);
 	p9_debug(P9_DEBUG_MUX, "wakeup: %d\n", req->tc.tag);
 	p9_req_put(c, req);
 }
diff --git a/net/9p/trans_fd.c b/net/9p/trans_fd.c
index eeea0a6a75b6..ee2d6b231af1 100644
--- a/net/9p/trans_fd.c
+++ b/net/9p/trans_fd.c
@@ -30,6 +30,7 @@
 #include <net/9p/transport.h>
 
 #include <linux/syscalls.h> /* killme */
+#include <linux/wait.h>
 
 #define P9_PORT 564
 #define MAX_SOCK_BUF (1024*1024)
@@ -728,6 +729,17 @@ static int p9_fd_cancelled(struct p9_client *client, struct p9_req_t *req)
 		return 0;
 	}
 
+	/* If the request is been sent to the server, we need to wait for the
+	 * job to finish.
+	 */
+	if (req->status == REQ_STATUS_SENT) {
+		spin_unlock(&m->req_lock);
+		p9_debug(P9_DEBUG_TRANS, "client %p req %p wait done\n",
+			 client, req);
+		wait_event(req->wq, req->status >= REQ_STATUS_RCVD);
+
+		return 0;
+	}
 	/* we haven't received a response for oldreq,
 	 * remove it from the list.
 	 */
-- 
2.37.3